The increasing demand for high-speed voice and data communications has led to an increased reliance on optical communications, especially optical fiber communications. The use of optical signals as a vehicle to carry information (often channeled) at high speed is preferred in many instances to carrying channeled information at other electromagnetic wavelengths/frequencies in media such as microwave transmission lines, coaxial cable lines, and twisted copper pair transmission lines.
Advantages of optical media include higher channel capacities (bandwidth), greater immunity to electromagnetic interference, and lower propagation loss. In fact, it is common for high-speed optical systems to have signal rates in the range of approximately several megabits per second (Mbit/s) to approximately several tens of gigabits per second (Gbit/s), and greater. However, as the communication capacity is further increased to transmit greater amounts of information at greater rates over fiber, maintaining signal integrity can be exceedingly challenging.
The emergence of optical communications as a useful vehicle for short and long haul data and voice communications has led to the development of a variety of optical amplifiers. One type of optical amplifier is the rare-earth metal optical amplifier. One such rare-earth metal optical amplifier is one based on erbium-doped silica fiber. The erbium doped fiber amplifier (EDFA) is widely used in the telecommunications industry.
While the EDFA has certain advantages, another type of optical amplifier is based on stimulated Raman scattering. To this end, when pump-light intensity within an optical waveguide (e.g., an optical fiber) becomes large, the glass molecules are excited into different vibrational states. In stimulated Raman scattering, the difference between two vibrational states can be used to amplify an optical signal, and thereby a device known as a Raman amplifier. Unlike rare-earth doped fiber amplifiers, the Raman amplifier does not require special dopants; Raman amplification may occur in the silica glass and in doped regions used to create the index differential to form the waveguide (e.g., Ge). Accordingly, the Raman amplifier can use the actual transmission waveguide as the gain medium.
The Raman amplifier typically uses a pump laser having a frequency that is separated by a predetermined amount from that of the optical signal frequency. This separation is normally on the order of approximately −13 THz (100 nm). Illustratively, for a 1.55 μm signal wavelength, a pump laser having a wavelength of 1.45 μm is used to induce stimulated scattering.
Raman amplifiers offer further advantages compared to rare-earth fiber amplifiers. For example, that the signal-to-noise ratio of a 15×100 km span Raman amplifier system is the same as a 6×100 EDFA system. This is due to the comparative reduction in cascaded noise build-up in the Raman system, enabling optical signals to pass more amplifying stages before it is sufficiently degraded to warrant detection and electronic regeneration. Because the electrical regeneration is required for individual wavelengths, and becomes more complex with increased transmission rate, it is far more costly than optical amplification, which is substantially bit rate independent, and capable of multi-channel amplification.
It is desired to provide Raman-based amplifiers that provide substantially flat gain over a relatively broad wavelength band, with low ripple, as well as other useful characteristics.